The juxtaposition of maternal and fetal tissues in the hemochorial placenta has led to speculation that maternal recognition of fetal Ags present on trophoblasts might play an important role in reproductive biology. We report here for the first time such recognition of trophoblasts by T lymphocyte hybridomas representative of certain cells present in the maternal decidua. Trophoblast recognition is TCR dependent and is mediated by members of the V gamma 1+ subset of gamma delta T lymphocytes, a population that we previously have shown to be associated with heat shock protein-60 reactivity. Recognition occurred in experiments in which trophoblast clones or freshly prepared trophoblasts were used and requires cell-cell interaction. Although the maternal-fetal immune relationship typically has been cast in terms of an allograft, the T cell recognition of trophoblasts as described herein is not MHC-restricted, inasmuch as freshly prepared trophoblasts from beta 2-microglobulin-deficient mice were found to be stimulatory. Furthermore, the trophoblast ligand that mediates this recognition is probably a conserved mammalian molecule, because a human trophoblast cell line is also stimulatory. Our findings suggest a novel form of T cell recognition, and may provide an enhanced understanding of the maternal-fetal immune relationship.